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Condensate Boiler - vertical versus horizontal venting

Hi All,

I'm working to replace my home heating system from oil to natural gas (high efficiency condensate boiler). As part of this I've been working on getting quotes from various companies and the data is flowing back. However I'm getting conflicting information that I was hoping experts here can help me understand.

Very short background: house is in the boston area, partially finished (often used) basement with multiple load bearing joists that will interfere with running exhaust piping in the inside of the house. The back of the house, where the heating system is currently located, has a wrap around elevated deck that I'm told will prevent simple exhaust through the basement wall since it would be under the deck. We use the deck extensively with a large collection of plants, etc so vent exhaust during the warm months (hot water) may be an issue if it kills the plants. Heating is a forced hot water system (heat and hot water).

My Question:

There are two possible venting options - up the current dedicated double insulated metal chimney (not brick - in a wood enclosure) used to vent the oil burner or go outside and run under the deck to the sides of the deck to keep them away from the plants.

One HVAC company has told me that venting under the deck (horizontal) could result in the vent pipe icing up over the winter possibly causing a blockage. Because of that they could not warrant the installation unless we went up the current chimney. This is a fairly high end - large well known company.

The other HVAC company has told me that venting up the chimney (vertical) might be problematic because if we had any issues with icing at the exhaust it would be inaccessible to us. They really really want to go under the deck.

Obviously such sharp contrasting information makes for a confused homeowner. I've tried to read a number of other postings and have read about the pitch of the vertical vent should be towards the device not towards the vent termination.

One complication - most of the vendors have indicated that using the existing chimney will pose a problem with respect to ensuring a waterproof terminator at the top since they would have to cut the existing vent stack off (once it comes out of the wood chimney box and clears the metal chimney cap) and replace it with some kind of homemade mechanism.

So a secondary question - is adapting a non-brick double insulated chimney by putting the pvc inside the existing metal chimney so unusual?

Any feedback would be hugely appreciated as I try to figure out what the "correct" approach for this job should be.

Well, PVC itself is not really legal for use with combustion venting--it is tolerated all over the country but does not have a legal leg to stand on. Now, you encase that plastic in a warm, confined space and the temps will rise further. PVC is only rated for about 140F and keep in mind that is not for heating equipment but hot water. Who knows what would happen to it as this is an unlisted, untested application. You cannot vent directly into the factory chimney. To modify the chimney termination would void the warranty and listing thus making it illegal. If you disharged into the existing termination through PVC, that would make the present termination an unlisted 'shroud', which would also be illegal. I would opt for venting out the back and deal with the issues there. HTH,

Pipe Type

Originally Posted by hearthman

Well, PVC itself is not really legal for use with combustion venting--it is tolerated all over the country but does not have a legal leg to stand on. Now, you encase that plastic in a warm, confined space and the temps will rise further. PVC is only rated for about 140F and keep in mind that is not for heating equipment but hot water. Who knows what would happen to it as this is an unlisted, untested application. You cannot vent directly into the factory chimney. To modify the chimney termination would void the warranty and listing thus making it illegal. If you disharged into the existing termination through PVC, that would make the present termination an unlisted 'shroud', which would also be illegal. I would opt for venting out the back and deal with the issues there. HTH,

Thanks for the reply, however this brings up a number of questions:

I was under the impression that PVC (or something similar) was normally used for the exhause of the NG condensate boilers. Is this untrue? If so what is used?

When you say "modify the chimney termination would void the warranty" do you mean the boiler warranty? To be clear no one is proposing the modification of the boiler chimney, only the existing (soon to be unused) metal insulated chimney. The modification would be to the existing cap to allow the new condensate boiler exhaust pipe to pass through it. This means the existing chimney would simply be a race way for the new exhaust to pass through.

I may be wrong, but I was under the impression that the exhaust was going to be both the intake for air as well as the exhaust. Would that not reclaim some of the heat such that the amount of heat in the old metal flue would be minimal?

I'm still a bit confused regarding the described icing issues. Is there anything to worry about having the exhaust vent vertically versus horizontally?

I have done both ways, hearth man always seems to have the best answers and addressed things I never thought of like like the chimney heating up and degrading the pipe. Some manufacturers won't allow insulating the vent pipe for that reason. I would run the exhaust to the end of the deck and turn it down, if its pitched correct you should be ok. The intake could be terminated at the wall. If the pipe freezes up just clear the ice and you will be fine, we get a few calls every year for that issue.

You should contact the manufacturer about outside venting. Many systems are ul listed and legal for use with PVC pipe whether it is a good idea or not. Some manufacturers have optional concentric pipes that have the exhaust go through the inner pipe and Intake around it. Polypropylene or pp pipe is the standard replacement for PVC for most countries but don't underestimate the ice dams in the exhaust it can kill you. A few failure would have to happen at once but it has happened that is why the manufacturer probably won't let you run horizontally outside.

PVC is listed by nearly every manufacturer of condensing equipment and UL listed and certified with same. The manufacturers of the pipe do not list it, primarily to avoid ambulance chasers. Properly installed (read manufacturer's installation manual for details) condensing boilers have proven perfectly safe for venting material when the appliances are properly installed and regularly maintained by trained and competent technicians.

I have worked on many condensing boilers going back to the first one and found disasters in venting, but very few could be blamed on the PVC. Look to the installer, the service technician (or lack thereof) and perhaps the original system designer.

As stated before, hearthman is usually the go to guy for venting questions. I am unclear what he is referring to as a 'shroud' but my thought is that you vent the boiler in schedule 80 PVC which is rated for 200 degrees. This is what most manufacturers are going to in the future in my opinion. Venting under a deck can and likely will cause the deck to rot out. I have seen this firsthand several times. Using the vertical metal piping as a chase way seems the way to go in my opinion but I will reserve the ability to change my mind after hearthman returns

The appliances themselves carry a listing but the PVC does not. In Canada, PVC can be listed to their S636 standard but that std. has not be accepted here in the US. I sit on the UL Standards Technical Panel that writes the UL listings. We cannot come to a consensus on glued polymeric venting for a number of reasons. There is polypropylene venting listed to UL 1738 that can be used for CAT IV venting. This venting, such as Inno Flue, uses a slip joint and is rated much higher temp. than PVC. One problem with PVC is not under normal operating conditions but adverse conditions. If a unit is accidentally overfired, the venting needs a reasonable degree of safety margin to withstand the elevated temps. Glued PVC is known to fail at temps above 140F.
The reason PVC is not listed is not within the ability of the mfrs. but due to the lack of a listing standard. The codes state all gas venting under positive vent pressure must be listed and installed to the listed instructions. A tankless water heater may, for example, use AL29-4c stainless steel venting listed to UL 1738. When it comes to appliances, the code contradicts itself by saying see the appliance listing for approved venting. A AHJ has the authority to approve or disapprove of any appliance installed in his jurisdiction. In the case with PVC, this is a new trend as we're seeing it being turned down more and more.
In the case of an incident where there was failure of the venting, the lack of a listing has been a key point. Part of listing polymeric venting includes specifying the primer and solvent cement, joint prep, etc, hangers, pitch, room for thermal expansion, etc, etc. If any of those steps are not followed then the venting would not meet the listing.
The PVC mfrs. are content selling their pipe to wholesalers who they know will sell their pipe to HVAC contractors who are using it for combustion venting. However, they all maintain instructions NOT to use their product for combustion venting applications. One other issue unresolved is how to test the installed piping prior to use. We conduct a pressure test on DWV pipe by code. The PVC pipe mfrs. all state flatly do NOT use air pressure to test their product or use their pipe for pressurized air applications.

I would NEVER flat out state anything is "safe". That is the most dangerous word in our lexicon and yes, I am CYA'ing here. The proving of the adequacy and competency of combustion venting is in incident/ loss investigation. Nobody is routinely inspecting each pipe joint and there is no accepted test modality for the pipe or its joints once installed. What is your definition Badger of "trained and competent" technicians"? What school teaches hands-on how to join each brand of PVC vent pipe? I would be interested in seeing a copy of this course curriculum and training materials.